1. Field of the Invention
The present invention relates to a reforming catalyst capable of steam-reforming a fuel such as dimethyl ether to produce hydrogen, and especially, to a reforming catalyst prepared by a sol-gel method and excellent particularly in low temperature activity.
2. Description of Related Art
Dimethyl ether (hereinafter, referred to as DME) is a fuel comparatively safer and having higher calories than methanol and the like, therefore, there is envisaged utilization of DME as a fuel of a fuel cell power generation system of small output used in a mobile power source for automobiles and the like. Fuel cell generation is actuated using hydrogen and oxygen as a fuel, therefore, when a DME fuel is used, a DME reforming catalyst producing hydrogen efficiently from DME is necessary.
Conventionally, Cu—Zn-based catalysts are widely known as a catalyst for obtaining a mixed gas rich in hydrogen by steam-reforming of methanol and DME. However, with these catalysts, a mixed gas after reforming has low hydrogen concentration and manifests low reforming efficiency.
As a method for preparing a reforming catalyst using a solid acid, an impregnation method as described in Japanese Patent Provisional Publication No. 2001-96159 is listed. However, in an impregnation method in which a carrier made of an inorganic oxide such as alumina (Al2O3) or silica (SiO2) is impregnated in a catalyst metal solution to support a catalyst metal, a catalyst metal is supported only on the surface of a catalyst layer, showing poor dispersion. Furthermore, in the impregnation method, decrease in a reforming ability is a caused sometimes by sintering (aggregation) of an active metal by heat.
In a co-precipitation method general as a catalyst preparation method, a catalyst is prepared by precipitation of a metal oxide or metal hydroxide by adding an acid or alkali to an aqueous solution of an inorganic metal salt. However, in the co-precipitation method, there occur sometimes problems where impurities contained in a raw material can not be excluded easily, that salts used in precipitation are liable to be incorporated as impurities and that a homogeneous catalyst can not be prepared easily due to irregularity in the pH in growth of a precipitation.
Furthermore, the present inventors have disclosed a Cu/Al2O3-based catalyst prepared by a sol-gel method in Japanese Patent Provisional Publication No. 2002-263504. This catalyst shows an excellent reforming ability at temperatures of 275° C. or more, however, further improvement has been required at lower temperatures 200° C. to 275° C.